In the last few years, the cost of electronic optical sensors has been lowered dramatically and digital imaging technology has been widely used in many different fields of applications. Also, the processing speed and power of microprocessors continue to increase and the cost of these microprocessors continues to decrease. However, scanners currently available on the market have not benefited greatly from the technical advances and price reductions.
In particular, one of many drawbacks associated with current scanning devices such as flatbed scanner is that scanning speed is very slow, as slow as 30 second per page. Current flatbed (glass-top) scanning devices, including flatbed scanners, the flatbed scanning portion of digital copiers and all-in-one machines, usually use a parallel moving scan head to capture the image of a stationary document. The scan head usually has a light source, mirrors, focusing lens and optical sensors. Among the many factors limiting the scanning speed such as line scan rate of optical sensor, data transferring rate, imaging processing speed, the speed of the mechanical movement of the scan heads is believed to be a major factor that limits the overall speed of current flatbed scanning devices. Therefore, eliminating the conventional scan head is the key to dramatically increase the scan speed.
In order to increase the scanning speed, alternative scanning mechanisms have been explored. U.S. Pat. No. 5,920,401 disclosed a compact document imager that may capture document image instantly. This device uses a large semi-transparent area optical sensor to do image capturing while illuminating the document using light from behind the optical sensor. Because light has to pass through the large area sensor, the sensor has low image capturing resolution. U.S. Pat. No. 6,747,764 B1 disclosed a “camera box” style device, which may capture document image instantly. It is understood that this device uses a digital camera, facing up, in a box that is topped with a transparent glass or plastic plate. The viewing angle of the field of view of the “camera” inside the box is limited. Therefore, it is difficult to capture the image of a large document without making the vertical dimension of the device relatively high. The bulky body of such a device makes it hard to be accepted as a document image-capturing device used in common offices.
Efforts have been made to overcome at least some of the aforementioned deficiencies. For example, in order to reduce the physical size of the scanner, multiple scan heads have been proposed, wherein each scan head provides a sub-image of the document. The new challenge is how to put all sub-images together. It is understood that U.S. Pat. No. 5,117,295 discloses a way to use “wires” placed between the imaging system and the document to serve as visual reference for sub-images alignment and for image reconstruction. Initialization and pre-scanning have to be used for sub-images alignment. In addition, “wires” are only used for alignment purpose. The sub-images taken by the disclosed imaging system are initially distorted and have un-even shading.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.